Under the current FCC rules, wireless providers are allowed to exclude a certain percentage of counties from their wireless Phase II accuracy calculations. They can do this for several different reasons:

If Carriers use a handheld-based solution (GPS triangulation), they can exclude counties for heavy forestation that may interfere with GPS signals. Up to 15% of the counties served by the carrier can be excluded for this reason.

If Carriers use a the network-based solution (cell tower triangulation), they can exclude counties where they have an insufficient number of cell sites, terrain makes triangulation difficult, where they have stand-alone sites, or where they have a "string of pearls", meaning a set of cell sites in a line along a road or highway, making triangulation impossible. Up to 40% of the counties served by the carrier can be excluded for these reasons.